FIG. 1 is a schematic illustration of a power amplification circuit 10 in a transmitter. As shown, the circuit 10 comprises a power amplifier 12, a balun 14 and an antenna 16.
The power supply connections of the amplifier 12 are indicated 18 and 20. As shown, the amplifier 18 has a power supply voltage of Vdd. The signal that is to be transmitted from the antenna 16 is presented as a differential signal across the input terminals 22 and 24 of the amplifier 12. The amplifier 12 amplifies that differential signal and outputs it across lines 26 and 28, which form the input to the balun 14.
A balun is a transformer that is designed to convert a differential signal into a single-ended signal (or vice versa in other scenarios). The balun 14 comprises a primary inductor 30 across which is applied the differential signal that is output by the amplifier 12. The balun 14 also comprises a secondary inductor 32 that is linked to the primary inductor 30 by a shared magnetic flux, indicated by the dotted arrows, such that a voltage is induced across the secondary inductor 32. The voltage that is developed across the secondary inductor 32 is the output signal of the balun 14 and is applied across the antenna 16 by means of lines 34 and 36. The voltage that the balun 14 produces across its output terminals 34 and 36 is the voltage that is applied across its inputs 26 and 28 scaled up by a factor of n. That is to say, the balun 14 has a transformation ratio of 1:n. Where the primary inductor has an inductance L1, the secondary inductor has and inductance L2 and the primary and secondary inductors 30 and 32 have a coupling factor of k, then n in the transformation ratio is given by:
                    n        =                              1            k                    ⁢                                                    L                2                                            L                1                                                                        [        1        ]            
If the impedance, from the point of view of the output of the amplifier 12, the effective impedance of the balun 14 and the antenna 16 is XE, then it can be shown that the output power POUT of the amplifier 12 is:
                              P          OUT                =                              2            ⁢                                                  ⁢                          V              dd              2                                                        ⁡                          [                              X                E                            ]                                                          [        2        ]            
In equation 2, [XE] is the real part of XE. (Classically, P=V2/R, but here the signal is differential so V=2Vdd and R=2[XE].)
Typically, it is required that the output power of a power amplifier in a transmit chain is adjustable. It will be apparent from equation 2 that this adjustability can be achieved in the case of amplifier 12 by altering Vdd. In the case where a conventional regulator is used to derive Vdd from a voltage VBAT supplied by a battery, the regulator could be controlled to adjust Vdd in a manner that provides the desired control over POUT. However, associated with the use of a regulator, there would be a power loss PLOSS of:PLOSS=IPA(VBAT−VDD)  [3]
In equation 3, IPA is the current consumed by the power amplifier 12.
Where power efficiency is a concern, a switched mode power supply (SMPS) could be used instead of a regulator. That is to say, rather than make the magnitude of Vdd continuously variable, Vdd can be periodically switched from a constant value to zero, for adjustable interludes. However, a SMPS will consume an undesirably large amount of space when implemented on a silicon chip and would still require a large off-chip inductor.